.TH std::rank 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::rank \- std::rank

.SH Synopsis
   Defined in header <type_traits>
   template< class T >              \fI(since C++11)\fP
   struct rank;

   If T is an array type, provides the member constant value equal to the number of
   dimensions of the array. For any other type, value is 0.

   If the program adds specializations for std::rank
   or std::rank_v
   \fI(since C++17)\fP, the behavior is undefined.

.SH Member constants

   value    the number of dimensions of T or zero
   \fB[static]\fP \fI(public static member constant)\fP

.SH Member functions

   operator std::size_t converts the object to std::size_t, returns value
                        \fI(public member function)\fP
   operator()           returns value
   \fI(C++14)\fP              \fI(public member function)\fP

.SH Member types

   Type       Definition
   value_type std::size_t
   type       std::integral_constant<std::size_t, value>

.SH Possible implementation

   template<class T>
   struct rank : public std::integral_constant<std::size_t, 0> {};

   template<class T>
   struct rank<T[]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};

   template<class T, std::size_t N>
   struct rank<T[N]> : public std::integral_constant<std::size_t, rank<T>::value + 1> {};

.SH Example


// Run this code

 #include <type_traits>

 int main()
 {
     static_assert(
             std::rank<int>{} == 0
         &&  std::rank<int[5]>{} == 1
         &&  std::rank<int[5][5]>{} == 2
         &&  std::rank<int[][5][5]>{} == 3 );

     [[maybe_unused]] int ary[][3] = {{1, 2, 3}};
     // The reason of rank of "ary[0]" is calculated as 0
     static_assert(std::rank_v<decltype(ary[0])> == 0);
     // is that rank cannot deal with reference type. i.e. int(&)[3]
     static_assert(std::is_same_v<decltype(ary[0]), int(&)[3]>);
     // The solution is to remove reference type
     static_assert(std::rank_v<std::remove_cvref_t<decltype(ary[0])>> == 1);
 }

.SH See also

   is_array           checks if a type is an array type
   \fI(C++11)\fP            \fI(class template)\fP
   extent             obtains the size of an array type along a specified dimension
   \fI(C++11)\fP            \fI(class template)\fP
   remove_extent      removes one extent from the given array type
   \fI(C++11)\fP            \fI(class template)\fP
   remove_all_extents removes all extents from the given array type
   \fI(C++11)\fP            \fI(class template)\fP
